The element of surprise as Rio’s authorities go in to take over another “favela” - or shanty town.

And yet the timing of this operation had been widely leaked. With good reason: to give the bad guys a chance to escape elsewhere - thus avoiding a chaotic shoot-out in which the people of this poor neighbourhood could be caught in the crossfire. It’s the voices of those people which are the most difficult to hear in the hours, days and weeks after one of these police “pacification” raids.

Say something positive about the new regime and risk being targetted by the ruthless drugs gangs who used to hold sway. Their gun arsenals may have been moved elsewhere and the Mr Bigs have scarpered ... but usually most of the gang infantry do not disappear.

Brazil's police dawn raids attempt to tame the favelas.

But say something negative about pacification and who’s to say what a cop might do (or threaten to do) in one of the favela’s many back alleys?

To someone from a culture where police are expected to act within the law (and, with occasional exceptions, the UK police do) this might seem far-fetched. To someone living in one of Brazil’s poor favelas it seems entirely plausible.

Brazil's shanty towns are known as favelas.

Brazil’s military police are the frontline protectors of law and order - the ones who patrol the streets. They are a relic of the two decades of dictatorship which ended in the mid-80s and are still organised into battalions.

There is corruption in some of these units. Some operate outside of the law, collaborating with the organised criminals they are meant to be arresting. Those cops who break the law do not often end up in court - police impunity is a big problem in Brazil.

Over the years cases of police brutality ... or disregard for the lives of poor people ... have been countless and well-documented. Only a few months ago, ITV News reported a police helicopter strafing a residential street with machine gun fire. The footage takes your breath away.

This weekend’s pacification operation was not the last. Thirty-five done so far and another five due by next year. One of the most challenging for the police will be Complexo da Mare which is a sprawling mass of favelas near the international airport, carved up into territories controlled by the warring drugs factions.

Brazil's police dawn raids attempt to tame the favelas.

In the last few months police have gone into Mare on raids to soften-up resistance ahead of a takeover. When I visited, local people (nothing to do with trafficking) complained of indiscriminate shooting from helicopters and police breaking into their homes and ransacking them.

In Rio’s largest shanty town, Rocinha which was pacified more than a year ago, investigators have in the last few days implicated ten military police officers in the torture and murder of a local man while complaints of threats and beatings against other members of the community are widely-reported here.

Cops I’ve spoken to say they get an unfair press. A group of young policemen patrolling a favela called Jacarezinho told me they sense hostility seeping out of the brickwork and fear ambush around every corner. “How would you feel?” they asked.

Some newer senior officers in the military police are pushing for a cultural change from within. Theirs is a tough challenge. Critics say an impossible one ... adding it’d be better to abolish the military police and start all over again.